The present invention relates generally to a fuel injection control in a fuel injection internal combustion engine which is associated with an automatic power transmission. More particularly, the invention relates to fuel cut-off control in a fuel injection control system for the purposes of fuel economy and emissions control.
In a fuel injection internal combustion engine, fuel cut-off is performed while the engine is decelerating or coasting in order to save fuel and reduce exhaust emission. On the other hand, even during engine deceleration or coasting, fuel cut-off is inhibitted and fuel injection must be performed under certain engine operating conditions for engine driving stability and prevention of engine stalling. In particular, in the range of relatively low engine speeds, fuel injection must be carried out even while the engine is decelerating or coasting.
For the internal combustion engine equipped with an automatic power transmission, it is difficult to prolong the period in which the fuel cut off is performed to satisfactorily save the fuel. In such an engine, the engine speed can more easily drop into the engine speed range requiring fuel supply than an engine with a manual power transmission, since the automatic power transmission has a torque converter which isolates the engine from a driving wheel shaft to transmit less driving force to the engine than is transmitted to an engine with a manual power transmission. This results in a shorter fuel cut-off period.
Furthermore, in the fuel cut-off control, hysteresis is provided between the fuel recovery engine speed at which fuel supply is resumed in order to prevent the engine from stalling and the fuel cut-off engine speed at which fuel cut-off is performed. Obviously, the fuel cut-off engine speed is higher than the fuel recovery engine speed. Therefore, once the engine speed drops to the fuel recovery engine speed, fuel cut-off cannot be performed until the engine speed reaches the fuel cut-off engine speed again. In an engine with automatic power transmission, the fuel recovery engine speed is preset to a comparably high engine speed in order to satisfactorily prevent the engine from stalling and, for the same reason, the fuel cut-off engine speed is preset to a higher engine speed than that required for engine warm up. As a result, the engine speed during deceleration of coasting can quickly pass the fuel recovery engine speed so that the fuel cut-off period is shortened and the engine speed will return rather slowly to the fuel cut-off engine speed, both of which unnecessarily increase fuel consumption.
On the other hand, it is well known that, with an automatic power transmission, the engine speed can be increased by shifting the transmission gear position to a lower gear during deceleration, similarly to kick-down during acceleration. By using this down-shifting of the transmission gear effectively, the engine speed can quickly reach the fuel cut-off engine speed to perform the fuel cut-off even after the engine speed has dropped to the fuel recovery engine speed. In order to effectively perform the down-shifting of the transmission gear, a down-shifting actuator for operating a down-shifting valve in the automatic power transmission for kick-down operation will be used even during engine deceleration. In this way, the period of time for which the engine speed falls within the fuel recovery range is shortened in order to decrease unnecessary consumption of fuel.